As information devices have rapidly come into widespread use, a wide variety of types of information is retained in information devices in recent years. Accordingly, a current very important issue is to establish a security technology that, for example, suppresses information from leaking from information devices. In view of this situation, various authentication technologies such as, for example, password authentication, biometric authentication and card authentication have been proposed. However, many existing authentication technologies carry out authentication processing only at the time of login, so these technologies have the common problem that if, for example, a third party makes an unauthorized access to an information device while the normal user is apart from the location at which the information device is installed, the unauthorized access is difficult to detect.
To address the above problem, there are continuous authentication technologies in which authentication processing is continued for the user even after the user has logged in. These technologies are disclosed in, for example, F. Monrose and A. D. Rubin, “Keystroke dynamics as biometrics for authentication,” Future Generation Comput. Syst., vol. 16, pp. 351-359, 2000, A. Altinok and M. Turk, “Temporal integration for continuous multimodal biometrics,” in Proc. Workshop on Multimodal User Authentication, 2003, pp. 131-137, and T. Sim, S. Zhang, R. Janakiraman, and S. Kumar, “Continuous verification using multimodal biometrics,” IEEE Trans. Pattern Anal. Mach. Intell., vol. 29, no. 4, pp. 687-700, April 2007. In typical continuous authentication, face authentication in which feature vectors are used is carried out. In face authentication, feature points, which are used as feature vectors in a face area corresponding to the user in an image, are stored in advance, and continuous authentication is achieved by detecting the user in the image according to a correspondence relationship between the stored feature points and feature points extracted during authentication.
Color histogram authentication, in which the color histogram of an image on which a user has been photographed is used, is also proposed in K. Niinuma, U. Park, A. K. Jain, “Soft Biometric Traits For Continuous User Authentication”, IEEE Transactions on Information Forensics and Security (TIFS), Vol. 5, No. 4, pp. 771-780, 2, 2010. Color histogram authentication has an advantage that robust continuous authentication is possible even if the orientation of the user changes when compared with other methods in which face authentication or the like is used. In color histogram authentication, the color histogram of, for example, an area (such as a body area) corresponding to the user in the image is stored in advance. The body area of the user in the image is detected according to the similarity of an extracted color histogram with the stored color histogram, and continuous authentication is achieved by tracking the movement of the body area.